The present invention relates generally to frequency locking for oscillators, and in particular, to the optical injection locking of solid state oscillators.
There are numerous military applications for microwave or millimeter-wave radiation from 10 GHz to 100 GHz. Such applications include a variety of imaging and seeking systems. A basic element of these systems is the solid state oscillator in which typically a field-effect transistor (FET), a transferred-electron diode (TED), or an impact ionization avalanche transit time (IMPATT) diode serves as the radiation source. The major requirements for these sources are stability both in phase and frequency, reliability, and capability for high-power operation. Also, compactness and cost can be important considerations depending on the application under consideration.
However, each solid state oscillator type mentioned above suffers from thermal instability resulting in frequency shifting. Also, some of these devices are limited in power output, so that techniques in power combining must be utilized. Power combining techniques require all sources to operate at the same frequency and to be correlated in phase in order to eliminate random fluctuations and frequency jitter.
A standard technique for stabilizing the frequency and phase of solid state oscillators comprises injection locking the oscillator to the frequency of an external signal. This technique is commonly used for frequency locking microwave IMPATT and FET transistor oscillators in order to reduce frequency jitter. The technique is also used for phase locking several slave oscillators to a single master oscillator. Recently, the injection locking of a microwave oscillator by illuminating the active region of the FET or IMPATT with an optical signal which is intensity modulated at the oscillating frequency or at one of its subharmonics was demonstrated. See H. W. Yen and M. K. Barnoski, Applied Physics Letters 32, 182, (1978); A. A. Salles and T. R. Forrest, Applied Physics Letters 38, 292 (1981); and H. W. Yen, Applied Physics Letters 36, 680 (1980). The advantages to such optical injection locking are, (i) high frequency carrier density fluctuations can be directly generated in the active areas of the device without electrical contacts and without the associated problems arising from stray impedance, reflections etc., and (ii) the distribution of the high frequency locking signal to many oscillators can be readily accomplished using optical fibers. However, using a standard current modulated GaA1As laser as the source of the injecting optical signal, the maximum .nu..sub.m at which the fundamental frequency locking can occur is restricted by the laser's limited frequency response. In essence, such laser diodes cannot be current modulated above 4 GHz due to the rate equations describing photon and carrier densities within the laser diode cavity. Thus, optical injection locking at frequencies in the 10-100 GHz range cannot be achieved in the prior art.